In an image pickup apparatus such as a digital still camera or a digital video camera, exposure controlling technique for automatically adjusting appropriate exposure has been conventionally known as the technique for preventing a phenomenon that an exposed image becomes utterly dark due to under-exposure or an exposed image becomes utterly light due to over-exposure.
This exposure controlling technique is adapted to determine an exposure value (i.e., the combination of an aperture opening degree and a shutter speed) at which the best exposure result can be obtained based on the luminance of a subject and the sensitivity of the charge pickup device (a charge-coupled device, or a CCD). In general, the average luminance of a photographic screen is calculated through a photographic lens, and then, the subject is exposed at a high exposure value in such a manner that an exposed image is prevented from being utterly light when the average luminance is high (i.e., the subject is light); and in contrast, the subject is exposed at a low exposure value in such a manner that the exposed image is prevented from being utterly dark when the average luminance is low (i.e., the subject is dark).
Here, systems for calculating the average luminance of the photographic screen include an averaging metering system for calculating the average luminance of the entire photographic screen; a center-weighted metering system for calculating the average luminance by placing prime importance on the luminance at the center of the photographic screen; a spot metering system for calculating the average luminance by using only the luminance of a specified portion of the photographic screen; a multi-pattern metering system for calculating the average luminance by using a pattern analysis based on pieces of luminance information on a plurality of portions, into which the photographic screen is divided; and the like.
As illustrated in FIG. 10A, when the average luminance of the photographic screen is calculated by using the luminance of a portion illuminated with a spotlight when the spotlight having a luminance much higher than that of the surroundings thereof is included within the photographic screen, a high average luminance is obtained, and therefore, an exposure value also becomes high. In an exposed image in this case, the image around the spotlight (a subject) becomes dark due to under-exposure although the portion of the spotlight can be prevented from becoming utterly light, as illustrated in FIG. 10B.
In order to prevent the entire exposed image from becoming dark by the under-exposure due to the brightness of a part of the photographic screen in the above-described case, high luminance cutting is carried out in the above-described metering system. A pixel having a luminance higher than a predetermined luminance threshold on the photographic screen (hereinafter referred to as “a high luminance pixel”) is subjected to high luminance cutting, so that the luminance of the high luminance pixel is converted into a predetermined low luminance equal to or lower than the luminance threshold, and then, the average luminance is calculated.
With this prior art, the average luminance of the photographic screen is calculated by using the low luminance after the conversion, so that an exposure value also becomes low. Consequently, it is possible to prevent the entire exposed image from being exposed at under-exposure due to the brightness of a part of the photographic screen.
However, since the luminance of the high luminance pixel is uniformly converted into the predetermined low luminance equal to or lower than the luminance threshold in the above-described prior art, there is an inevitable limit to correct exposure control.
Namely, in the above-described prior art, the luminance of the high luminance pixel is converted into the predetermined low luminance equal to or lower than the luminance threshold even when a high luminance portion consisting of the high luminance pixels on the photographic screen is to be exposed as a main subject, and therefore, the exposure value also becomes low. Therefore, when the image is exposed at the low exposure value, hollows occur at the high luminance portion, thereby arising a problem that the correct exposure control cannot be carried out while the high luminance portion is accurately regarded as the main subject.